This invention concerns means for adjusting or selecting various operating characteristics of an implantable cardiac pacer without physically contacting the pacer.
Implantable cardiac pacers are essentially resin encapsulated pulse generators which can be connected to the heart to apply stimulus pulses thereto. In some subjects asynchronous pulse generators are used. This type of pulse generator applies stimulus pulses at a constant rate and each pulse has a substantially constant width and amplitude. Asynchronous pacers are usually used in subjects who have complete ventricular conductive block wherein no natural electric stimuli are produced by the heart.
The standby pacer is another type that is commonly used. In this type, there is a pulse generator and a sensing circuit connected to the heart. When a natural electric stimulus occurs on the heart, the sensing circuit causes the pulse generator to be inhibited for a time interval. The sensing circuit may be made to exhibit hysteresis. That is, when a natural stimulus is sensed, the next pulse to be generated by the generator may be delayed for a relatively long interval and if a natural pulse does not occur within that interval the generator will turn on and pulse at shorter intervals or at a higher rate until the next natural pulse, if any occurs. Sensing circuits also usually have a refractory period during which they will not respond to noise or other electric signals on the heart following the moment after a natural signal is sensed.
It is desirable that the foregoing functional parameters of the pacer be as compatible as possible with the physiological needs of each cardiac patient in which a pacer is implanted. Stated in another way, it is desirable to optimize the stimulus pulse rate, amplitude and width for each patient to meet physiological needs and also to conserve battery energy and in many cases it would be desirable to be able to select the refractory interval, the hysteresis interval and sensitivity of the sensing circuit after the pacer has been surgically inplanted.